Large sliding doors in London extensions

1. Choosing the system

Start from the opening, not the brochure. Three things decide the right system: the width, the height and how the client wants to use the opening.

• Up to about 4 m wide and 2.4 m high, with moderate glass weights: a good inline slider is often enough and costs less.
• Over 4 m, or taller than 2.5 m, or triple-glazed: panel weights climb quickly, and lift & slide hardware is designed for that. It also seals better, which matters on exposed or noisy sites.
• If the client wants the whole wall open in summer, a bi-fold opens the most width. Show them an elevation of the closed doors first, because that's how they'll see it most of the year.

Glass weight is the number that drives most decisions. Glass weighs about 2.5 kg per square metre for every millimetre of thickness. A triple unit with 16 mm of glass therefore weighs about 40 kg/m², so a 1.4 × 2.5 m panel carries roughly 140 kg of glass before the frame. Our sizing tool does this sum for you.

2. The head: beams and deflection

In a typical rear extension the door sits under a steel beam that carries the house above. Sliding doors are unforgiving here. If the beam deflects onto the head of the frame, panels bind, seals open and locks stop lining up.

• Ask the structural engineer to design the beam to the deflection limit given for the door system, not just a general span ratio. We supply that limit at Stage 3.
• Allow a deflection gap and a sliding head detail between the beam and the frame, so the structure can move without loading the door.
• Remember that post-free corners move the corner load into the structure. These need a cantilever or a hidden post, agreed early.
• Check the fixing zone. The frame needs a solid, continuous fixing at head and sill, not just packers on a steel flange.

3. The floor: recess, drainage and thermal line

A flush threshold is the main reason clients choose lift & slide, and it is a floor detail first. Three things have to be drawn before the slab is poured:

1. The recess. The frame sits down in the structure. The recess depth depends on the system and the floor build-up, so take it from the system details, not a rule of thumb.
2. Drainage. With no upstand, water on the outside has to go somewhere. Use a linear drain against the frame, paving that falls away, and an outlet that can't block with leaves.
3. The thermal line. The insulation has to run continuously under and around the frame. If it doesn't, the threshold becomes a cold bridge with condensation risk, especially with underfloor heating near the door.

4. Getting the glass in

This is the London problem nobody draws. Most terraces have no rear access, and a 3 m, 250 kg panel will not go through a Victorian hallway.

• Measure the route from the kerb to the opening: front door, hallway, stairs, ceiling heights and turns. Panels usually travel on edge, so ceiling height matters as much as width.
• Plan the lift. Panels over about 150 kg need mechanical handling, such as a vacuum lifter or glazing robot. Heavier panels or blocked routes may need a crane over the house.
• Allow for licences. A crane, scaffold or skip on the highway usually needs a licence from the borough, and often a parking bay suspension. Build that time into the programme.
• Consider the sequence. Sometimes the best answer is to install the glass before the house is finished, or to split one panel into two.

5. Building Regulations, in plain English

These summaries apply to England. Always check the current Approved Documents and any local requirements.

Part L: conservation of fuel and power

For new or replacement doors and windows in existing homes, the limiting U-value is 1.4 W/m²K. In new dwellings the limiting value is 1.6 W/m²K, but the overall SAP calculation usually pushes the target lower. For extensions, the simple route assumes the total area of windows, doors and rooflights is no more than 25% of the extension floor area, plus the area of any openings the extension removes. Larger glazing is allowed, but it then has to be justified with a SAP-based calculation, so bring the energy assessor in early.

Part O: overheating (new dwellings)

Large south and west-facing glazing is the classic overheating risk. Solar control glass with a lower g-value, external shading and enough openable area all help. The assessment needs the glass data, which we provide.

Part K: protection from impact

Doors and side panels are critical locations, so the glass must be safety glass: toughened or laminated. Large sliders are glazed this way as standard, but check guarding wherever there's a drop outside.

Part M: access (new dwellings)

Accessible thresholds should have a total upstand of no more than 15 mm. A flush or low threshold detail meets this.

Part Q: security (new dwellings)

Ground-floor and easily reachable doors in new dwellings must be shown to resist attack, usually by testing to PAS 24. Ask for the test evidence for the system and the configuration you're using.

Part F: ventilation

Sliders rarely take trickle vents. Background ventilation for the room has to come from windows, rooflights or mechanical ventilation, so plan it early.

Source: Approved Documents L, O, K, M, Q and F, GOV.UK. Contains public sector information licensed under the Open Government Licence v3.0. Summary only, not legal advice.

Stage 3 checklist

• Panel layout and weights checked against the system limits
• Deflection limit agreed with the structural engineer, and head detail drawn
• Corner or pocket structure agreed, if used
• Slab recess, drainage and continuous thermal line drawn
• Threshold type chosen for exposure and Part M
• Delivery route measured and lifting method agreed
• Glazing percentage and Part L route agreed with the energy assessor
• Part O, K and Q requirements confirmed for the building type
• Background ventilation planned elsewhere